Methods for joining two blanks and blanks and products obtained

ABSTRACT

Methods for joining a first blank and a second blank, at least one of the first and second blanks comprising at least a layer of aluminum or of an aluminum alloy or a layer of zinc or of a zinc alloy. The method comprises selecting a first portion of the first blank to be joined to the second blank, and selecting a second portion of the second blank to be joined to the first portion, and welding the first portion to the second portion. The welding comprises using a filler metal laser beam and a welding laser beam, and displacing both laser beams in a welding direction to melt and mix a filler wire material with the melted portions of the two blanks. The present disclosure further relates to blanks obtained by any of these methods and to products obtained from such blanks.

This application is a continuation of International Application No.PCT/EP2016/081493 filed Dec. 16, 2016, which claims the benefit ofEuropean Patent Application EP15382641.7 filed on Dec. 18, 2015. Theentire contents of both applications are incorporated herein byreference.

The present disclosure relates to methods for joining two blanks, andmethods for obtaining products after joining two blanks. The presentdisclosure further relates to products obtained by or obtainable by anyof these methods.

BACKGROUND

The development of new materials and processes for the production ofmetal pieces with the aim of reducing component weight at a low cost isof utmost importance in the automotive industry. In order to achievethese objectives, the industry has developed ultra-high-strength steels(UHSS) which exhibit an optimized maximal strength per weight unit andadvantageous formability properties. These steels are designed to attaina microstructure after heat treatment, which confers good mechanicalproperties and makes them especially suited for the hot stamping processused to form steel blanks into particular automobile parts. Since duringthe hot stamping process the blank is subjected to aggressiveatmospheres, the steel is usually coated to avoid corrosion andoxidation.

In an attempt to minimize the weight of components while respectingstructural requirements, so-called “tailored blank” techniques may beused. In these techniques, components may be made of a composite metalblank which is obtained by welding several blanks with optionallydifferent thicknesses, different materials, size and properties. Atleast theoretically, using this kind of technique the use of materialmay be optimized. Blanks of different thickness may be joined or a steelblank may be joined with a blank of a different material for example,using the specific properties of each material where they are needed.

These blanks may be welded “edge to edge” (“butt-joining”). Theseso-called tailored blanks are designed to be hot stamped and afterwardsbe assembled to form automotive parts. Tailored welded blanks may beused for structural components such as doors, B-Pillars, beams, floor,bumpers, etc.

Similarly, “patchwork” blanks are known, in which several blanks are notnecessarily welded “edge-to-edge”, but instead partial or completeoverlaps of blanks may be used.

An example of steel used in the automotive industry is 22MnB5 steel. Thecomposition of 22MnB5 is summarized below in weight percentages (rest isiron (Fe) and impurities):

C Si Mn P S 0.20-0.25 0.15-0.35 1.10-1.35 <0.025 <0.008 Cr Ti B N0.15-0.30 0.02-0.05 0.002-0.004 <0.009

Several 22MnB5 steels are commercially available having a similarchemical composition. However, the exact amount of each of thecomponents of a 22MnB5 steel may vary slightly from one manufacturer toanother. Usibor® 1500P is one example of a commercially available 22MnB5steel manufactured by Arcelor. The composition of Usibor® is summarizedbelow in weight percentages (rest is iron (Fe) and impurities):

C Si Mn P S Cr Ti B N 0.24 0.27 1.14 0.015 0.001 0.17 0.036 0.003 0.004

In other examples, the 22MnB5 may contain approximately 0.23% C, 0.22%Si, and 0.16% Cr. The material may further comprise Mn, Al, Ti, B, N, Niin different proportions.

Various other steel compositions of UHSS may also be used in theautomotive industry. Particularly, the steel compositions described inEP 2 735 620 A1 may be considered suitable. Specific reference may behad to table 1 and paragraphs 0016-0021 of EP 2 735 620, and to theconsiderations of paragraphs 0067-0079. In some examples the UHSS maycontain approximately 0.22% C, 1.2% Si, and 2.2% Mn.

Steel of any of these compositions (both 22MnB5 steel such as e.g.Usibor® and the other compositions mentioned or referred to before) maybe supplied with a coating in order to prevent corrosion and oxidationdamage. This coating may be e.g. an aluminum-silicon (AlSi) coating or acoating mainly comprising zinc or a zinc alloy.

Patchwork blanks and tailored blanks may also be used or useful in otherindustries.

Usibor® 1500P is supplied in a ferritic-perlitic condition. Themechanical properties are related to this structure. After heating, hotstamping, and subsequent rapid cooling (quenching), a martensiticmicrostructure is obtained. As a result, maximal strength and yieldstrength increase noticeably.

As mentioned before, Usibor® 1500P may be supplied with analuminum-silicon (AlSi) coating in order to prevent corrosion andoxidation damage. However, this coating has a significant effect relatedto its weld behavior. If Usibor® 1500P blanks are welded without anyfurther measures, aluminum of the coating may enter into the weld areaand this can cause an important reduction of the mechanical propertiesof the resulting component and increase the possibility of fracture inthe weld zone.

In order to overcome this problem a method was proposed inDE202007018832 U1 which consists in removing (e.g. by laser ablation) apart of the coating in an area close to the welding gap. This method hasthe disadvantage that an additional step is needed for the production ofthe (tailored) blanks and components and that in spite of the repetitivenature of the process this additional step requires a complex qualityprocess with an elevated number of parts which are to be scrapped. Thisentails an increase of the cost of the welding step and limits thecompetitiveness of the technology in the industry.

US20080011720 proposes a process for laser welding at least one metalworkpiece by a laser beam, said workpiece having a surface containingaluminum, characterized in that the laser beam is combined with at leastone electric arc so as to melt the metal and weld said workpiece(s). Thelaser in front of the arc allows the use of a flux-cored wire or thelike containing elements inducing the gamma-phase (Mn, Ni, Cu, etc,)favourable to maintaining an austenitic structure throughout the meltedzone.

However, problems related to the only partial dilution of the fillermaterials along the depth of the welding zone have been found whichresult in a reduced welding strength. Furthermore, the filler materialmay not distribute homogeneously in the welding zone. This may causematerial accumulation (“bumps”) in certain areas thus affecting locallythe behaviour of the welding zone. That is, the mechanical properties ofthe welding zone may vary. Another problem may be that the fillermaterial may need to be preheated before applied because the electricarc may not be capable of melting it otherwise.

Herein a blank may be regarded as an article which has yet to undergoone or more processing steps (e.g. deformation, machining, surfacetreatment or other). These articles may be substantially flat plates orhave more complicated shapes.

In examples of the welding methods described herein the aforementioneddisadvantages are avoided or at least partially reduced.

SUMMARY

In a first aspect, the invention provides a method for joining a firstblank and a second blank, at least one of the first and second blankscomprising at least a layer of aluminum, of an aluminum alloy, of zincor of a zinc alloy. The method comprises selecting a first portion ofthe first blank to be joined to the second blank, and selecting a secondportion of the second blank to be joined to the first portion, meltingthe first portion to the second portion, while supplying a filler wireto a weld zone using a first and a second laser beams. The first laserbeam melts the filler wire in the weld zone during welding, and thefirst portion and the second portion of the blanks are melted and mixedwith the melted filler wire using the second laser beam.

By using two laser beams, each one for a different purpose, it ispossible to adjust the characteristics of the beams to their intendeduse. Such characteristics may be the power of the laser beam or thedimension of the spots. For example, the filler wire may require adifferent power to melt than the portions of the blanks. Another examplemay be the width of the weld zone compared with the size of the fillerwire; each one may require a different spot size.

Without being bound to any theory, it is believed that with the twolaser beams it is possible to generate or improve a Marangoni effect inthe welding zone (in the melted pot).

The Marangoni effect (also called the Gibbs-Marangoni effect) is themass transfer along an interface between two fluids due to surfacetension gradient. In the present case, the Marangoni effect is a fluidflow created in the “weld pool” due to a temperature distribution in theweld pool. The surface tension is dependent on temperature andtherefore, these temperature differences create a surface tensiongradient on the surface of the weld pool. That is, the melted part ofthe substrate and the melted part of the filler material that are closerto the surface—and are therefore hotter—will be drawn from the region oflower surface tension (higher temperature) to the region of highersurface tension (lower temperature). As a result, a fluid flow (fluidbeing the melted part of the substrate and the melted part of thefiller—reinforcement—material) is created in such a way that the heightdistribution and the penetration of the filler material in the weldingzone is increased. The fluid flow may resemble a spiraling downwardmovement from the upper hotter layers of the welding zone towards itslower cooler layers.

In some examples, using the second laser beam may comprise displacingthe second laser beam in an oscillating manner to mix the first portionand the second portion of the blanks with the melted filler wire. Theoscillating movement of the laser beam may cause the materials in theweld pool to mix more homogeneously as a result (or in part as a result)of the Marangoni effect. Such an oscillating movement may comprisedifferent beam motions such as a spiraling or circular movement around acentral point, a wobbling movement or a weaving (zig-zag) movement alongthe weld direction, or a combination thereof.

In some examples, using the second laser beam may comprise generating atwin-spot to melt the first portion and the second portion and to mixthe first portion and the second portion of the blanks with the meltedfiller wire. Two sub-beams may be generated with twin-spot laser optics,each sub-beam generating one of the two spots of the twin-spot. The useof a twin-spot may also mix the materials in the weld pool morehomogeneously, again (partially) as a result of the Marangoni effect.

In some examples, the first laser beam used for melting the filler wiremay have a spot having a size corresponding (e.g. equal or greater) tothe filler wire diameter. Therefore, it may accurately and preciselyconcentrate all its energy for the purpose of melting the filler wire.The second laser beam used for melting the first portion to the secondportion and for mixing the melted filler wire may generate a spot or atwin-spot having a size corresponding to a size of the weld zone. Morespecifically, in case of a single spot, a size (e.g. width) of the weldzone may be equal or greater than the size of the spot. In case of atwin-spot, a size (e.g. width) of the weld zone may be equal or greaterto the aggregate size of the two spots of the twin-spot. The size of theweld zone may be a size of the desired welding. It may correspond toknown tolerances of the blanks so that any gaps between the blanks to beappropriately filled during the welding.

In some examples, the two laser beams may be generated by a single laserhead. This may facilitate alignment and improve the speed of thewelding.

In some other examples, the first laser beam may be generated by a firstlaser head and the second laser beam may be generated by a second laserhead. This may allow for easier individual control of the beamcharacteristics (e.g. shape, power) of the two beams.

In some examples the two laser beams may generate spots arrangedsubstantially in line with a welding direction. The spot or spotsgenerated by the second laser beam may precede or follow the spot of thefirst laser beam. Therefore, the first laser beam may generate one spotand the second laser beam may generate one or more spots, and the spotsof the first and the second laser beam may be arranged substantially inline with a welding direction.

In some examples, when the second laser beam is used to generate atwin-spot, the spot of the first laser beam may be arranged before,after or between the spots of the twin-spot generated from the secondlaser beam. Furthermore, the two spots of the twin-spot may be arrangedperpendicularly to the welding direction. Alternatively, the two spotsof the twin-spot may be arranged collinearly to the welding direction.

In some examples, when the spots are arranged perpendicular to thewelding direction, the two spots of twin-spot of the second laser beammay precede or follow the spot of the first laser beam. Alternatively,the spot of the first laser beam may be arranged collinearly between thespots of the twin-spots.

In some other examples, when the spots are arranged collinearly to thewelding direction, the two spots of the twin-spot of the second laserbeam may precede or follow the spot of the first laser beam.Alternatively, the spot of the first laser beam may be arrangedcollinearly between the spots of the twin-spots.

The choice of spot arrangement may depend on the characteristics of thecoating, the filler material, the desired welding or of a combinationthereof.

In some examples, the first and second blanks might be butt-jointed, thefirst portion might be an edge of the first blank and the second portionmight be the edge of the second blank. Specifically, a square butt-joint(without machining or beveling of the edges) may be used. Morespecifically, a closed square butt weld may be used.

In some examples, the first and/or the second blank comprises a steelsubstrate with a coating comprising the layer of aluminum or of analuminum alloy or the layer of zinc or of a zinc alloy. In some examplessuch steel substrate of the first and/or the second blank might be anultra-high strength steel, in particular a 22MnB5 steel.

In another aspect, a method for forming a product is disclosed. Themethod comprises forming a blank including a method of joining a firstand a second blank according to any of the methods described herein,heating the blank, and hot deforming and subsequent quenching of theheated blank.

In yet another aspect, a blank as obtainable by any of the methodsproposed herein is disclosed.

In yet another aspect, a product as obtained by a method for forming aproduct as proposed herein is disclosed.

Different lasers may be used for laser welding such as Nd-YAG(Neodymium-doped yttrium aluminum garnet) and a CO2 laser withsufficient power. Nd-YAG lasers are commercially available, andconstitute a proven technology. This type of laser may also havesufficient power to melt the portions (together with the arc) of theblanks and allows varying the width of the focal point of the laser andthus of the weld zone. Reducing the size of the “spot” increases theenergy density.

Different filler wires may be used, according to any requirements of thewelding zone, as the power of the filler wire melting laser may beadjusted to the requirements of the filler wire (e.g. meltingtemperature). The filler wire used may comprise gammagenic elements tostabilize the austenitic phase. Austenitic stabilizing elementscounteract the ferrite stabilizing effect of Al or Zn, thus minimizing(or avoiding) ferrite in the final weld joint. According to this aspect,aluminum (or zinc) may be present in the weld zone, but it does not leadto worse mechanical properties after hot deformation processes such ashot stamping when the filler wire comprises gammagenic elements, whichstabilizes the austenitic phase. These gammagenic elements areintroduced in the weld zone and mixed with the melt, and as aconsequence austenite (gamma phase iron, γ-Fe) may be obtained byheating. During rapid cooling (quenching) after a hot deformation, amartensitic microstructure which gives satisfactory mechanicalcharacteristics may thus be obtained

There is thus no need to remove an aluminum, aluminum alloy, zinc orzinc alloy layer, such as was proposed in some prior art methods. Whene.g. coated steel blanks are to be welded, this may be done quicker andcheaper since an intermediate process step is not necessary anymore.

Gammagenic elements are herein to be understood as chemical elementspromoting the gamma-phase, i.e. the austenite phase. The gammagenicelements (or “austenitic stabilizer elements”) may be selected from agroup comprising Nickel (Ni), Carbon (C), Manganese (Mn), Copper (Cu)and Nitrogen (N). Although the addition of “ferrite stabilizer elements”may counteract the action of “austenitic stabilizer elements”,optionally these “ferrite stabilizer elements” can still be suitablecomponents when other factors are also taken into account for thecomposition of the filler. For example, for promoting hardnessMolybdenum (Mo) could be a suitable element and e.g. for corrosionresistance Silicon (Si) and Chromium (Cr) could be suitable components.

Aluminum alloys are herein to be understood as metal alloys in whichaluminum is the predominant element. Zinc alloys are herein to beunderstood as metal alloys in which zinc is the predominant element.

Preferably, the amount of gammagenic elements in the filler wire issufficient to compensate for the presence of alphagenic elements such asCr, Mo, Si, Al and Ti (Titanium). Alphagenic elements promote theformation of alpha-iron (ferrite). This may lead to reduced mechanicalproperties as the microstructure resulting after hot stamping andquenching may comprise martensite-bainite and ferrite.

In some examples, the filler may contain a austenite stabilizingelements and may have a composition in weight percentages of 0%-0.3% ofcarbon, 0%-1.3% of of silicon, 0.5%-7% of manganese, 5%-22% of chromium,6%-20% of nickel, 0%-0.4% of molybdenum, 0%-0.7% of Niobium, and therest iron and unavoidable impurities.

In other examples, the metal filler material may be stainless steel AlSi316L, as commercially available from e.g. Hoganäs®. The metal filler mayhave the following composition in weight percentages: 0%-0.03% carbon,2.0-3.0% of molybdenum, 10%-14% of nickel, 1.0-2.0% of manganese, 16-18%chromium, 0.0-1.0% of silicon, and the rest iron and unavoidableimpurities.

Alternatively 431L HC, as commercially available from e.g. Hoganäs® maybe used. This metal filler has the following composition in weightpercentages: 70-80% of iron, 10-20% of chromium, 1.0-9.99% of nickel,1-10% of silicon, 1-10% of manganese and the rest impurities.

Further examples may use 3533-10, as further commercially available frome.g. Hoganäs®. The filler has the following composition in weightpercentages: 2.1% carbon, 1.2% of silicon, 28% of chromium, 11.5% ofnickel, 5.5% of molybdenum, 1% of manganese and the rest iron andimpurities.

It was found that the presence of nickel in these compositions led togood corrosion resistance and promoted the austenite formation. Theaddition of chromium and silicon aids in corrosion resistance, andmolybdenum aids in increasing the hardness. In alternative examplesother stainless steels may also be used, even UHSS. In some examples,the filler may incorporate any component providing higher or lowermechanical characteristics depending on circumstances.

Additionally, it has been found that a filler of these mixtures leads tovery satisfactory mechanical properties of the final work product, i.e.after hot stamping and quenching. Also, other fillers can be used.

In a second aspect, the present disclosure provides a method for forminga product comprising forming a blank including a method of joining afirst and a second blank in accordance with any of the herein describedwelding methods and subsequently heating the blank, and hot deforming ofthe heated blank and final quenching. Heating may include heat treatmentin a furnace prior to deformation. Hot deforming may include e.g. hotstamping or deep drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting examples of the present disclosure will be described in thefollowing, with reference to the appended drawings, in which:

FIGS. 1a-1d schematically illustrate examples of joining two blanks;

FIGS. 2a-2c schematically illustrate example arrangements for a weldinglaser beam and a filler wire melting beam according to variousimplementations; and

FIGS. 3a-3f schematically illustrate relative positions of welding laserbeams and filler wire melting beams.

FIG. 4 is a flow diagram of a method of joining blanks.

DETAILED DESCRIPTION OF EXAMPLES

FIGS. 1a-1d schematically illustrate examples of methods of joiningblanks. In FIG. 1a a first portion or region A1 of a first blank A is tobe joined to a second portion or region B2 of a second blank B. In thisexample, the two blanks are to be butt-joined, i.e. an edge-to-edgewelding, specifically with straight edges (without specialshaping/bevelling of the edges).

In this example, both blanks A and B may be of coated steel, such ase.g. Usibor® 1500P. Both blanks may comprise a steel substrate 1 uponwhich a coating 2 may be provided. The coating applied in this exampleis aluminum-silicon (Al187Si10Fe3). Due to the process of application ofthe coating, the resulting coating may have a metal alloy layer 4 and anintermetallic layer 3 as illustrated in FIG. 1b -1 d.

FIGS. 1b-1d schematically illustrate a cross-sectional view along theplane defined by the line x-y and the corresponding top view accordingto some examples of dual laser welding. Such plane defined by the linex-y corresponds to the welding beam C, i.e. the line where the edge ofblank A contacts the edge of blank B. In these examples, blanks A and Bmay comprise a steel substrate 1 with a coating 2, which may have ametal alloy layer 4 as the outermost layer and an intermetallic layer 3arranged between the steel substrate 1 and the metal alloy layer 4. Whenblanks A and B are welded, the coating layer and the steel substrate ofthe welded portions of blanks A and B, and the filler are mixed in thewelding beam. Thus, after welding, the welding beam does not comprise adefined coating layer. In these examples, the arrow WD indicates thewelding direction in the top view.

FIG. 1b further illustrates a cross-sectional view along the planedefined by the line x-y and the corresponding top view of the method ofjoining according to an example of dual laser welding. Schematicallyillustrated is a cross-sectional and top view of a filler metal meltinglaser 20 having a laser head 21 from which a first laser beam Ll exits.A filler wire 25 may be used as welding material. Also schematicallyillustrated is a laser welder 30 having a laser head 31 from which asecond laser beam L2 exits.

In a dual laser welding process, two laser beams collaborate to form aweld zone 40. In this example, the first laser beam L1 (directly) meltsthe filler wire. The second laser beam L2 melts portions of the blanksin a weld pool substantially where the two blanks are to be welded. Themelted filler wire is directed in the—common—weld pool and at the sametime the melted filler wire mixes with the melted portions of theblanks. As the filler wire melts, any gap between the blanks may befilled and a weld may be created.

FIG. 1b further illustrates a top view of the weld zone 40 created inthe zones to be welded of the blanks A and B. Laser beam spot S1corresponds to the spot created by the first laser beam L1, while laserbeam spot S2 corresponds to the spot created by the second laser beamL2.

In the example of FIG. 1b , the second laser beam L2, the laser welderbeam, may be moveable in a wobbling manner to mix the material in theweld pool as a consequence of the Marangoni effect. As the meltedportion of the blanks comprises steel substrate material as well ascoating material, mixing the weld pool ingredients may avoid any harmfuleffects attributable to the Al alloy coating and, therefore, mechanicalproperties of the welded zone may not be affected.

It may be seen that in this case, there is no need for removing thecoating of the steel substrates prior to welding, as the homogeneousmixing of the materials along the whole thickness of the blanksmitigates any harmful effects of the coating thus simplifying andspeeding up manufacture. This may bring about a substantial costreduction. At the same time, a filler wire of suitable composition mayensure that good mechanical properties are obtained after the standardheat treatment for Usibor® and after hot deformation processes such ashot stamping.

A standard treatment for Usibor® blanks would be to heat the obtainedblank in e.g. a furnace to bring about (among others) austenization ofthe base steel. Then the blank may be hot stamped to form e.g. a bumperbeam or a pillar. During rapid cooling after a hot deformation,martensite which gives satisfactory mechanical characteristics may thusbe obtained. The standard treatment is not affected in any manner by themethods of joining proposed herein. In particular, thanks to theelements of a suitable filler wire (i.e. filler wire with gammagenicelements) that are supplied into the weld zone, a martensite structurecan also be obtained in the area of the weld, in spite of the presenceof aluminum.

FIG. 1c further illustrates a cross-sectional view along the planedefined by the line x-y and the corresponding top view of a method ofjoining two blanks according to another example of dual laser welding.Schematically illustrated is a filler metal melting laser 20 having alaser head 21 from which a first laser beam L1 exits. A filler wire 25may be used as welding material. Also schematically illustrated is alaser welder 30 having a laser head 31 from which two sub-beams L2 a andL2 b exit. The laser head 31 may comprise twin-spot laser optics.

In this example of dual laser welding process, the laser beams alsocollaborate to form a weld zone 40. The first laser beam L1 melts thefiller wire 25 similarly as in the example discussed with reference toFIG. 1b . The two sub-beams, L2 a and L2 b, generate a twin-spot thatmelts portions of the blanks in a weld pool substantially where the twoblanks are to be welded. The melted filler wire is directed inthe—common—weld pool and at the same time the melted filler wire mixeswith the melted portions of the blanks. The twin-spot may warrant themixing of the melted filler wire material with the melted portions ofthe blanks without any wobbling of any of the sub-beams L2 a and L2 b tobe required.

FIG. 1c further illustrates a top view of the weld zone 40 created inthe zones to be welded of the blanks A and B. Laser beam spot S1corresponds to the spot created by the first laser beam L1, while laserbeam spot S2 a and S2 b corresponds to the spots created by thesub-beams L2 a and L2 b respectively.

FIG. 1d represents a variation of the example of FIG. 1b , having asingle laser head 51 and a single laser melting the wire and welding. Inthis example the melting and welding laser 50 has a single laser head 51from which a first laser beam L1 and a second laser beam L2 exit.

FIG. 2a schematically illustrates a top view of a method of joining twoblanks according to an example. A first blank A is to be joined to asecond blank B along a weld seam C, wherein a first laser beam spot S1may be responsible for melting a filler wire 25 material in the weldseam C zone and a second laser beam spot S2 may be responsible formelting a portion of the first blank A and a portion of the second blankB as well as mix the melted filler wire material with the meltedportions of the blanks. The perforated line circles indicate thecircular movement of the second laser beam in order to homogeneously mixthe melted materials. FIG. 2b schematically illustrates a weavingmovement of the laser beam spot S2 while FIG. 2c schematicallyillustrates a wobbling movement of the laser beam spot S2. The selectionof movement may depend on weld zone characteristics.

In all the examples illustrated herein so far, blanks in the shape offlat plates are joined together. It should be clear that examples of themethods herein disclosed may also be applied to blanks of differentshapes.

FIGS. 3a-3f schematically illustrate the relative positions of the spotsgenerated from the first and second laser beams when a twin-spot laserbeam is used for melting the portions of the blanks and for mixing themelted portions of the blanks with the melted filler wire. The arrowindicates the welding direction. In FIGS. 3a-3c the three spots arearranged collinearly along the welding direction. In FIG. 3a the spotsS2 a and S2 b of the twin-spot precede the spot of the filler wiremelting beam. In FIG. 3b the spot of the filler wire melting beam S1precedes the spots S2 a and S2 b of the twin-spot. In FIG. 3c the spotS1 of the filler wire melting beam is interpolated between the two spotsS2 a and S2 b of the twin-spot. In FIG. 3d the spots S2 a and S2 b ofthe twin-spot precede the spot S1 of the filler wire melting beam.However, in this case, the two spots of the twin-spot are arrangedperpendicularly to the welding direction. In FIG. 3e , the two spots S2a and S2 b of the twin-spot are arranged also perpendicularly to thewelding direction, but, contrary to the arrangement of FIG. 3d , theyfollow the spot S1 of the filler wire melting beam. Finally, in FIG. 3f, the three spots are arranged along a direction perpendicular to thewelding direction where the spot S1 of the filler wire melting beam isinterpolated between the two spots S2 a and S2 b of the twin-spot.

When a twin-spot is used, the two spots may also induce or improve asimilar Marangoni effect and the elements of the welding zone may againbe homogeneously distributed with the austenite stabilizing elements inthe filler reaching the bottom part of the weld. Therefore, the aluminummay not lead to worse mechanical properties in the welding zone afterhot deformation processes such as hot stamping.

The percentage of ferrite and austenite depends on the amount ofaluminum. Adding these austenite stabilizing stainless filler materialsmay increase the mass content of aluminum necessary for starting theferrite phase. In other words, thanks to the filler, more aluminum maybe allowed in the weld area while still maintaining the desiredmechanical properties, i.e. while still ensuring the presence ofaustenite. Thus, the influence of the aluminum in the welding area maybe minimized and a weld joint with good mechanical properties may beobtained.

FIG. 4 is a flow diagram of a method of joining blanks according to anexample. In box 105, a first portion of a first blank to be joined to asecond blank may be selected. The first blank may comprise at least alayer of aluminum or of an aluminum alloy or a layer of zinc or of azinc alloy. In some examples, the first blank might comprise a steelsubstrate with a coating comprising the layer of aluminum or of analuminum alloy or the layer of zinc or of a zinc alloy. In someexamples, the steel substrate may be an ultra-high strength steel, inparticular the steel may be a boron steel.

In box 110, a second portion of a second blank to be joined to the firstportion may be selected. The second blank may also comprise at least alayer of aluminum or of an aluminum alloy or a layer of zinc or of azinc alloy. In some examples, the second blank might comprise a steelsubstrate with a coating comprising the layer of aluminum or of analuminum alloy or the layer of zinc or of a zinc alloy. In someexamples, the steel substrate may be an ultra-high strength steel and inparticular a boron steel.

In box 115, using a laser welding beam, the first portion and the secondportion of the blanks may be melted in a weld zone. In box 120, a fillerwire may be supplied and melted to the weld zone using a filler wiremelting laser beam. The filler wire melting laser beam corresponds to afirst laser beam. Such first laser beam is arranged to melt the fillerwire in the weld zone. The laser welding beam may correspond to a secondlaser beam. Using such second laser beam may comprise displacing thesecond laser beam in an oscillating manner or using a twin-spot laser.

In box 125, the melted portions of the blanks and the melted filler wireare mixed in the weld zone to produce a weld. By mixing the filler alongthe whole weld zone, i.e. along the whole thickness of the blanks,mechanical properties of the weld can be improved.

Good mechanical properties are obtained, where two Usibor® 1500P blankswere welded by dual laser welding with the use of a filler wire meltinglaser beam and a welding laser beam. Particularly, a high tensilestrength is obtained when fillers containing austenite stabilizingmaterials are used. The tensile strength obtained could be compared withan unwelded Usibor® products and a welded 22MnB5 uncoated boronproducts.

These good mechanical properties may be obtained using a relatively highwelding speed, improving the manufacturing processes and reducing thewelding time. Welding speed from 5-12 m/min may be achieved in variousexamples.

Although only a number of examples have been disclosed herein, otheralternatives, modifications, uses and/or equivalents thereof arepossible. Furthermore, all possible combinations of the describedexamples are also covered. Thus, the scope of the present disclosureshould not be limited by particular examples, but should be determinedonly by a fair reading of the claims that follow.

1. A method for joining a first blank and a second blank, wherein one orboth of the first blank and the second blank comprises a steel substratewith a coating of aluminum or aluminum alloy, and wherein the first andthe second blanks are square butt-jointed, a first portion being an edgeof the first blank and a second portion being an edge of the secondblank, the method comprising: selecting the first portion of the firstblank to be joined to the second blank, and selecting the second portionof the second blank to be joined to the first portion; melting the firstportion and the second portion, while supplying a filler wire to a weldzone using a first laser beam and a second laser beam, wherein the firstlaser beam melts the filler wire in the weld zone during welding, andthe first portion and the second portion of the blanks are melted andmixed with the melted filler wire using the second laser beam.
 2. Themethod according to claim 1, wherein using the second laser beamcomprises displacing the second laser beam in an oscillating manner tomix the first portion and the second portion of the blanks with themelted filler wire.
 3. The method according to claim 1, wherein usingthe second laser beam comprises using a twin-spot laser beam to melt thefirst portion and the second portion and to mix the first portion andthe second portion of the blanks with the melted filler wire.
 4. Themethod according to claim 1, wherein the first laser beam generates aspot having a size substantially corresponding to a diameter of thefiller wire and the second laser beam generates a spot or a twin-spothaving a size substantially corresponding to a size of the weld zone. 5.The method according to claim 1, wherein the first and second laserbeams are generated by a single laser head.
 6. The method according toclaim 1, wherein the first laser beam is generated by a first laser headand the second laser beam is generated by a second laser head.
 7. Themethod according to claim 1, wherein the first laser beam generates onespot and the second laser beam generates one or more spots and the firstand second laser beams generate spots arranged substantially in linewith a welding direction.
 8. The method according to claim 1, whereinusing the second laser beam comprises generating a twin-spot comprisingspots, and wherein the spots of the twin-spot are arranged substantiallyperpendicularly to a welding direction.
 9. The method according to claim8, wherein the spots of the twin-spot either precede or follow the spotof the first laser beam in the welding direction.
 10. The methodaccording to claim 1, wherein using the second laser beam comprisesgenerating a twin-spot comprising spots, wherein the spots of thetwin-spot and the spot of the first laser beam are arranged collinearly,and wherein the spot of the first laser beam is arranged between thespots of the twin-spot.
 11. The method according to claim 1, wherein thefiller wire comprises gammagenic elements.
 12. The method according toclaim 1, wherein the steel substrate of one or both of the first and thesecond blank is an ultra-high strength steel.
 13. A method for forming aproduct, the method comprising: forming a blank according to the methodof claim 1 for joining a first blank and a second blank, heating theblank, and hot deforming and subsequent quenching of the heated blank.